A power screwdriver typically includes a driving motor, a screwdriver bit drivingly rotated by the driving motor, and a transmission system interposed between the motor and the screwdriver bit to transmit rotational torque to the screwdriver bit.
The transmission system is provided with clutch means for interrupting torque transmission when a predetermined torque load is exerted on the screwdriver bit. The clutch means includes an annular cam formed on its one side with a cam path, a plurality of clutch balls held in contact with the cam path, a compression spring disposed along the axis of the screwdriver bit to urge the clutch balls against the cam, and an adjusting member for adjusting the compressive force of the compression spring. With this arrangement, the cam and the clutch balls are engaged with one another to allow torque to be transmitted through the transmission system until a predetermined torque is applied between the cam and the clutch balls. When the applied torque exceeds such a predetermined torque, the clutch balls are rotated relative to the cam so that no rotational torque is transmitted through the transmission system.
The transmission system is contained within a cylindrical housing. The adjusting member is in the form of a cup and includes a cylindrical wall and an end wall connected to the front end of the cylindrical wall. A female thread is formed on the inner surface of the cylindrical wall. A male thread is formed on the outer surface of the housing and threadingly engaged with the female thread. Threaded adjustment of the adjusting member serves to regulate the compressive force of the compression spring which is held in contact with the end wall of the adjusting member. The front end of the transmission system acts as a bit holder or chuck for holding the screwdriver bit. The end wall of the adjusting member has an opening through which the bit holder forwardly projects from the adjusting member. Such a conventional power screwdriver is disclosed in Japanese patent application publication No. 61-270080.